1. Field of the Invention
The invention relates to a pulse-width modulator wherein through a gate circuit a square-wave oscillation having a fixed duty cycle and a fixed frequency is pulse-width modulated in proportion to the value of an input signal.
2. Description of the Related Art
Pulse-width modulators are, for example, used in clocked and controlled direct voltage converters, therein converting an analog voltage available as a setting quantity into a square-wave signal having a variable duty cycle. With the pulse-width modulated signal the switching transistor of the converter is opened or closed depending on the length of the pulses.
The German Patent DE-PS No. 32 27 296 discloses a pulse-width modulator comprising a sawtooth oscillator, two comparators, an AND-gate and amplifiers. One comparator compares the analog input signal to the sawtooth voltage and the other comparator compares the sawtooth voltage to a time-constant voltage. The time-constant voltage is derived from the supply voltage by a voltage divider. The output voltages of the comparators are applied to the inputs of the AND-gate, whose output signal represents the pulse-width modulated signal. The duty cycle of the output signal varies between a maximum value to be adjusted by the voltage divider and zero value, always according to the amplitude of the continuous input signal.
As the known pulse-width modulator requires two comparators having short-process delay times and a sawtooth generator having a linear ramp voltage and as these building blocks consist of bipolar transistors, it will have a relatively large power dissipation. Therefore, it is, for example, unsuitable for use in clocked direct voltage converters having a switching rate of approximately 20 kHz, if extremely low internal losses are additionally required.